1. Field of the Invention
This invention relates to film loop take-up mechanisms and more particularly to a novel film loop apparatus having positive demand drive and associated sensors therefor for transport-in-storage of a continuous film loop external to a film printer.
2. Brief Description of the Prior Art
To increase the efficiency of printing multiple copies of motion pictures, microfilm, and other photographic masters onto photographic duplicating film, a device is needed to transport-in-storage all of the continuous-web loop external to the printer. Typically, in the past, rollers were mounted in pairs on a panel and the film looped over the rollers in a seesaw pattern and then back to the printer. More recently loop cabinets have been produced with multiple shafts, each carrying numerous rollers. The film traveled from the printer to the cabinet, and was threaded in loops around top and bottom roller pairs, first across the first pair of shafts, then across the second, and so on until the cabinet was filled or all of the film was stored. To accommodate for various film lengths, the upper shafts mounting the rollers were fixed at the top of the cabinet and were all driven by a common motor. The lower shafts were mounted in a frame. A means was provided to adjust the space between upper and lower rollers by raising or lowering the lower frame. Sometimes this was done with the aid of springs, weights, or levers; and sometimes by a manually-controlled motor.
To assist in film transport through the cabinet, the metal rollers were mounted on roller bearings. A motor turned all shafts on the top frame in the direction of film travel. The turning shafts caused the rollers to lose their resistance to being turned by the moving film passing over them. Without this aid, film tension would become too great and the film would break.
Equipment of the type described above is currently in common use throughout photographic and motion picture laboratories. However, there are numerous drawbacks to the loop cabinets now in use. For example, loading the cabinet presents problems because all roller shafts are motorized from a single motor and individual shafts cannot vary their speeds to take up more film when film length is increased or decreased. In addition, since the rollers are metal, they have a great deal of inertia and cannot rapidly change speed to match start and stop acceleration and deceleration. Further, since the rollers are not driven, slack accumulates quickly, and the operator must manually make adjustments, and sometimes move the cabinet closer to or further from the printer to compensate for film length changes. In addition, present loop cabinets have a maximum capacity often less than the length of the master to be accommodated. All of these factors contribute to limiting the throughput of the printer, thereby reducing the production efficiency of the laboratory.